BMW Alternator and BMW Starter:
Understanding Your BMW's Electrical System

Today's auto electrical systems are getting more intricate and are stressing the limits of current technology but they are basically the same design as 30 years ago. What can today's do-it-yourselfer do to keep from being electrically-challenged in the middle of nowhere? A quick overview of your BMW's electrical system would be a good start. The major components of your BMW's electrical system are explained below, along with some troubleshooting tips.

Your BMW Battery

The battery is a storage device, currently 12 volts, used to start the engine and help operate the electrical accessories installed on your BMW. The battery consists of six cells of stacked positive and negative lead plates, separated by insulators and immersed in electrolyte, which is a water and sulfuric acid mixture. Each of the six cells "produces" 2.1 volts for a total of 12.6 volts (although it's actually stored energy, not produced).

The chemical reaction created between the lead plates and the electrolyte creates dangerous, explosive gases that vent through the battery cover vents. Care should always be taken when charging or jump starting low batteries and whenever working under the hood of the car. Always keep the battery clean to allow proper ventilation.

Some batteries use a gel to replace the electrolyte for a somewhat safer battery and the use of "maintenance-free" batteries has eliminated the need for refilling the electrolyte. However, all batteries lose charging capacity over time through the loss of the electrolyte, deterioration of the plates and chemical breakdown of the connections.

Whenever your BMW exhibits symptoms of a charging or starting system failure, the most basic test that should be performed first is a visual and voltage test of the battery. Knowing the exact condition of the battery is the best way to know whether or not to suspect other system components and can prevent the unnecessary installation of a BMW starter or alternator, which cannot repair the vehicle until the battery is up to snuff.

To analyze the condition of a battery, very little high-tech equipment is really needed. While there is excellent equipment available for big $$$$, all that is really needed is a good DVOM (Digital Volt Ohm Meter), a battery charger, some patience and a little common sense.

A quick way to tell if a battery needs recharging is by observing the "eye" on the battery and being able to determine what the color of the eye is telling you. Below is a chart of various battery manufacturers that use a colored eye, as well as what the different colors mean:

Brand Names

Charge Indicator Color

Battery Charge Condition

Atlas, Delco, Delkor



Rocket, Solite, Yuasa

Blue or Green


Atlas, Delco, Delkor


Needs Charging

Rocket, Solite, Yuasa


Needs Charging

Atlas, Delco, Delkor

Clear or Light Yellow

Low fluid level; battery should be replaced

Rocket, Solite, Yuasa


Low fluid level; battery should be replaced

Before replacing any battery due to a failure, have the battery and the car's electrical system tested for proper functioning. Replacing a dead battery only to have another a few weeks later due to a weak alternator or a parasitic drain on the system is not only frustrating but can be expensive and dangerous as well.

An average battery should last 3-5 years in average climates but those in high heat or extreme cold areas may last only 2-3 years. The replacement battery should always have the same or higher CCA rating (cold cranking amps) as the original and be of the same or compatible "group size" to fit the battery tray and cable connections.

Your BMW Alternator

The BMW alternator produces electricity used to maintain battery storage charge and to help run all the electrical accessories, including the ignition and the engine control systems. It is belt-driven by the engine and produces an alternating current (AC), which is converted internally to 12 volts direct current (DC) by the diode bridge or rectifiers.

AC current cannot be stored but is much more efficiently produced, which is why cars no longer use generators but use alternators and convert the electricity to DC. Most alternators now use internal voltage regulators to maintain the proper system voltage, from 12.6-14.5 volts. You should check your BMW's repair manual or with your local dealer to obtain the exact proper voltage for your BMW.

Contrary to popular belief, a BMW alternator does not constantly produce electricity. It cycles on and off as demand goes up and down. The battery buffers it from the electrical demands of the car, and it only works enough to maintain system voltage. At peak efficiency and for longest life, the alternator should be charging no more than about 50% of the time.

However, with the demands placed on the system by heat and by extraneous electrical devices (i.e., high powered stereos, running lights, etc. - see Power Demand Chart below), the alternator and battery are stressed even further. The average BMW alternator now is lucky to last 3-4 years, which is why a prematurely dead battery may actually be caused by a dead alternator.

Typical Power Demands on a car's charging system are as follows:


Electrical Draw

Rear Window Defogger

25 amps

High Blower

20 amps

Headlamps (low)

15 amps

Windshield Wipers

6 amps


6 amps

Brake Lights

5 amps


77 Amps

And this doesn't include amperage requirements for items like cell phones, laptops, CD players, boom boxes or additional lights.

Understanding the importance of having an operational warning light for the alternator (idiot light) is crucial to catching problems early. In most modern systems, the electrical current passing through the filament of the warning light is what energizes a circuit in the alternator to start charging.

This signal light is linked directly to the alternator through its terminal (#1, I, L, D+, etc. depending on the brand) and functions slightly differently on different manufacturers.

To check the warning light circuit, turn the ignition switch to the "on" position without cranking or starting the engine; if the idiot light does not come on, remove the plug from the alternator and ground the wire that terminates to the #1, I, L or D+ terminal (depending on manufacturer). If the light comes on, the wiring is okay but the alternator is defective; if the light still does not come on, the wiring to the light circuit and the bulb should be checked.

Don't forget to check the fuse that controls the light circuit, too. This fuse could be labeled differently in various cars. It could be labeled "charging", "regulator", "meters", "gauges" or "engine". In some cars, if the fuse is out, the idiot light will come on but may not go off. In others (like GM), a burned out fuse may make the warning light work in reverse order; that is, when the key is on, the light is off but as soon as the engine starts and the alternator starts charging, the light will come on.

Checking out these simple circuits first can greatly reduce your troubleshooting time and unnecessary replacement of your alternator.

As we delve further into charge light indicators, we find that in some cases it is normal for the charge indicator light to come on when nothing is wrong with the alternator.

According to information published by GM, any car may have a low voltage reading or lights that dim when electrical loads are heavy at idle. Furthermore, this condition is normal and no repairs should be attempted unless a fault has been found.

For clarification, as a car idles for extended periods of time during high heat conditions, a number of things happen that contribute to "lowered" alternator output that coincides with physics and the design of the alternator:

1.  As heat within the alternator increases, the electrical resistance in the alternator also increases, which reduces the alternator's charging capacity.

2.  As temperature rises, the voltage setpoint of the regulator is lowered to reduce the chance of overcharging or "boiling" the battery.

3.  Newer designed alternators have a "delay/soft start" built into the regulator circuit. This delays the load being placed upon the engine when starting up from a stop, so that the smaller engines in use today are not loaded down upon acceleration due to charging demands. This can delay the charging by up to 15 seconds.

With the alternator's capacity for charging reduced by heat and other factors, an alternator may only be able to produce up to 70% of its rated output under these conditions. So an alternator rated for 100 amps may only be able to produce 70 amps when hot at idle when there is 77 or more amps of demand on it.

If it can be considered normal for warning lights to glow while a healthy alternator is running, how do you know if the alternator is really good or if there are other problems lurking around?

A thorough diagnosis is always the best route to determining whether or not the alternator is at fault, but there are times when diagnosis time is short and you still need a positive identification of the problem. Cases like this require a foolproof tool to speed things up. In the case of Delco CS series alternators, there is a tool available from Kent-Moore tools (J-41450-B), which isolates the alternator from the car's wiring harness and lets you see if the alternator is at fault or if there is a wiring problem elsewhere within the car's wiring harness. The best thing about this tool, besides being compact and handheld, is that it doesn't require any interpretation of data by the operator. The little light on the unit lights or doesn't light depending upon whether the alternator is good or not.

Unfortunately, while this is a great little piece of equipment and there are other similar tools available, once you determine that the alternator is not the culprit, you still have to fix the electrical problem.

Your BMW Starter

The other major electrical component in your BMW's electrical system is used only a few times a day but is the single largest power user and most critical to your BMW's operation - its starter. A BMW starter is simply a DC motor that turns the engine crankshaft through the flywheel, starting the combustion process by creating compression within the cylinders. Voltage to the starter is supplied directly from the battery and is controlled by a relay and/or solenoid operated from the key switch inside your BMW.

Starters can be of varying types and designs - gear-reduction types for higher torque, permanent-magnet types to reduce size and weight, or just plain, old-fashioned heavy starters. But whatever the type, they all function in the same basic way.

A slow cranking engine may be a sign of a bad starter and with age, that's more and more likely. But on most cars today, it's due to low battery voltage, poor electrical connections at the battery or a failed relay or fusible link.

Most starters will easily outlast a new vehicle warranty if it's not overused, if good connections are maintained and if it's not overheated through dirt and grime buildup.

Starting your BMW with the major components turned off (like the AC compressor, blower motor and high-powered stereos) will greatly ease the load on the starter. In fact, most new cars have "lock out" relays that will not allow the AC compressor and alternator to turn on until after the vehicle has been started. But turning these power-hogs off before shutting off your BMW is always a good precaution.

Although the starter drive, or "Bendix" as it was commonly referred to, can be replaced separately from the starter assembly, it's rarely recommended anymore. Failure of any part is due to age, usage and heat stress, to which the entire BMW starter has also been subjected, so that other parts are just as old and stressed. It's quite common to replace the starter drive only to have to buy another starter in a few months because the brushes wore out, a magnet broke, the solenoid failed, among other common mishaps. Replace the BMW starter as a unit and have the electrical system checked at the same time to prevent further problems.

Other Troubleshooting & Maintenance Tips
For Your BMW's Electrical System

Your BMW's electrical system should be completely checked and tested every two years or whenever serviced for any type of driveability problem. Many problems associated with day-to-day driveability are caused by voltage variations and must be the first step in troubleshooting any problem. This is due to the use of computerized controls in most cars these days and even quite minor voltage changes can alter the controls.

Your BMW's electrical system must be load tested to certain standards, which can be simulated by turning on all the accessories and lights for simple voltage drain but that is not an all-inclusive test. Measuring circuit loads with an ammeter, circuit voltage drops with a DVOM, variable circuit load testing, etc. is the only way to fully check function. With electrical systems operating at 80%-100% of capacity nowadays (see Power Demand Chart), it is crucial that it be up to standards.

The average do-it-yourselfer would have little need to purchase the more critical test equipment, so if voltmeter testing doesn't pinpoint the problem, get a thorough checkup done from an auto electrical technician who knows your BMW's system.

A complete and thorough test involves much more than sticking a voltmeter on the battery and the average do-it-yourselfer does not have the test equipment nor does he/she need it. You should allow a professional to do this test. As a good starting point, though, most major chain auto parts stores offer a free service test (in the hopes of selling you a battery or alternator) that should be good enough to notify you of any major problems. Most early problems start from poor electrical connections due to loose connections and/or buildup of corrosion, especially at the battery posts. Keep that battery clean!

Idiot Light Electrical Problems By Car Make

Here are a few quirky electrical problems - organized by car make - to help you troubleshoot your import car's electrical system idiosyncrasies:

BMW #1: The BMW starter on many late model BMWs incorporates a one-way clutch that allows the engine to spin faster than the starter during engagement. While this makes for smooth, quiet starting and gear engagement, it also provides opportunity for problems.

Many BMWs requiring a Bosch reman starter SR0445X are experiencing multiple starter failures due to the starter motor remaining engaged after the engine has started and the ignition key has been released.

You guessed it . . . there's nothing wrong with the starter - it's the ignition lock cylinder assembly. The mechanical portion of the ignition switch gets hung up in the start position and doesn't return to the run position. When this happens, the one-way clutch in the starter allows the starter to continue spinning with the engine; prolonged use in this condition will overheat the starter causing permanent damage as well as draining the battery. To correct this condition, the entire lock cylinder, steering lock and electrical switch unit must be replaced.

BMW #2: BMW has recognized that a similar problem exists with 1989 635s (AL145X and AL0740X Bosch reman alternators), where the battery runs down due to a 300 milliamp draw on the battery caused by the trunk light switch. Instead of coming up with a new switch, BMW suggests that the trunk light switch that is connected to terminal "30" (hot at all times) be rerouted to terminal "R", which is only "hot" when the ignition switch is sent to Accy, Run or Start.

In order to do this, the red/green wire should be removed from the glove box light, then taped securely back into the wiring harness. Next, a length of .75mm wire approximately 700mm long should have a connector and connector housing attached to one end. This end should then be connected to the glove box light, where the red/green wire was removed.

The other end of the 700mm long wire should be connected to the violet/white wire from the radio using a crimp type connector. This may sound too simple and maybe it is . . . you have to remove the glove box trim and the radio to perform this wiring maneuver.

BMW #3: 1992 BMW 318i models with a central locking system may exhibit frequently discharged batteries due to dome lights sporadically remaining on. This is due to faulty door lock striker plates that have the light contact switch built in. Seems that some versions of these switches were leaky and when water, dirt, salt, etc. leaked in, the contacts in the switch were bridged causing the lights to stay on even though the car was not running. Rather than needlessly replace the BMW alternator (Bosch reman alternators AL0153X, AL0152X, AL0151X), look at the striker plates first. If they have an alphabetic letter lower than "L", they need to be replaced with an improved switch (51 21 8 105 511).

A Few Important Things to Remember

Heed these tips and you're well on your way to extending the life of your BMW's electrical system components:

Tip #1: Always keep your battery and its connections clean to avoid clogged battery cover vents and overtaxing your starter. This will also allow for proper ventilation of dangerous, explosive gases from your battery.

Tip #2: When replacing your battery, always buy one of the same or higher CCA rating (cold cranking amps) as the original battery and make sure it's the same or compatible "group size" to fit your battery tray and cable connections.

Tip #3: Due to the varying nature of car electrical systems, never jump start your BMW using another car that is running. Use the other vehicle's battery power alone to start it because a 14.5 volt running system (i.e., GM) can seriously damage a 12.6 volt system (i.e., BMW) due to the overvoltage.

Tip #4: Start your BMW with the major electrical hogs turned off - A/C, stereo, etc. - to ease the load on your battery and starter and extend their lives.

Tip #5: Have your BMW's electrical system completely checked and tested every two years or whenever you have it serviced for any type of driveability problem.

Your BMW Electrical System Maintenance Shopping List

Here is a list of electrical system parts to check and maintain, plus some common tools, testers and spare parts to keep in your trunk or garage:

Check & Maintain - Replace When Necessary:
Battery (cleanliness, connections and up to charge)
BMW Alternator (charging properly, connections and belt tension)
BMW Starter (cleanliness and good connections)
All cables & connections, especially ground (remove rust & corrosion)

Common Tools, Testers ∓ Spare Parts to Keep on Hand:
DVOM (digital volt-ohmmeter)
Ammeter (for testing circuit load)
Battery post cleaner brush or tool
Battery charger (w/overcharge protection)
Spare fuses of all sizes in car
Crimp-on wire connector kit w/crimping tool
Quality electrical tape (for good isolation and insulation)
Repair manual with electrical schematic


Don't Forget:

Repair articles are added regularly.
Come back often to check for new maintenance topics.

These repair tips are designed only as a starting point.
Please seek the assistance of a professional BMW mechanic
for all repair problems beyond your capabilities.

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